1. Field of the Invert%ion
The present invention relates to a novel magnetic optical disc having a diameter of less than 80 mm and capable of storing at least 130 megabytes of compressed digital signals. More particularly, the invention relates to a magnetic optical disc constituted by a transparent substrate having a double refraction retardation of within .+-.50 nm over an area having an inner diameter of at least as small as 32 mm, whereby a sufficiently large recording area is provided on a small-diameter magnetic optical disc.
2. Description of the Prior Art
The optical data recording method offers a number of advantages including easy-to-handle non-contact recording, and reproduction, high resistance to scratch or soil over the recording surface of the disc, and a huge recording capacity, tens or hundreds of times that of the currently used magnetic recording method. These favorable characteristics have prompted the optimal recording method to be implemented in the form of the compact disc and video disc for recording digitized audio and video signals. Furthermore, the optical recording method is increasingly finding its way into such applications as mass files of coded and image information.
The magnetic optical recording medium for use with the optical recording method is a transparent substrate illustratively made of polycarbonate resin and having an optical data recording layer thereon. The substrate is subject to various requirements when formed into a disc. One such requirement is a low level of double refraction (i.e., misalignment between the incident light for reading data from disc and the reflected light, primarily due to the internal distortion of the transparent substrate). Other major requirements include: high ability to be stamped, smooth substrate surface, low contamination, and low skew level of the formed surface.
In particular, the magnetic optical disc has one mandatory requirement to meet: because tiny revolutions of the magnetic optical disc on its plane of polarization under an irradiated laser beam are read as signals, the double refraction occurring within the transparent substrate must be minimized.
Against this background, advances and improvements have been made in the technology of injection molding by leading manufacturers in the field. The headway now makes it possible to form transparent substrates whose recording areas are substantially free of double refraction for such applications as compact discs and video discs.
FIG. 6 is a schematic cross-sectional view of a typical conventional metal mold for molding disk substrates. Referring to FIG. 6, what follows is a description of how the representative prior art method of injection molding is illustratively implemented.
In the metal mold shown in FIG. 6, the cavity to be filled with the plastic resin is primarily constituted by a disc-shaped part 103 and a sprue 104, the disc-shaped part 103 being sandwiched by a fixed-side mirror plate 101 and a moving-side mirror plate 102, the sprue 104 being located at the mold center.
The fixed-side mirror plate 101 and the moving-side mirror plate 102 are each provided with temperature controls such as heating medium passages 105 and 106. The heating medium (usually hot water or oil) flowing through the passages controls the temperature of the mirror plates and 102.
The moving-side mirror plate 102 is equipped with a stamper, not shown, for stamping pits and guide grooves onto the signal area of the disc. The stamper is supported by having its pre-punched inner circumference sandwiched by both an inner circumference stamper stop 107 and the moving-side mirror plate 102. The stamper has its outer circumference supported by an outer circumference stamper stop 108.
At the fixed-side mold center is a die 109 engaged with a sprue bush 111 via a sprue bush cap 110. At the center of the sprue bush 111 is the sprue 104 that introduces the molten resin from inside a molding machine cylinder 112 into the disc cavity through a nozzle 113. It is through the sprue 104 that the molten resin goes into the cavity.
Between the sprue bush 111 and the sprue bush cap 110 is a cooling means, not shown, independent of the temperature controls for the mirror plates 101 and 102. This cooling means illustratively comprises grooves provided over the outer circumference of the sprue bush 111, the grooves allowing a cooling medium to flow therethrough. Generally, an O-ring 114 is used to seat the cooling medium.
One reason that this portion needs to be cooled independently of other parts is that the sprue 104, being thicker than the disc-shaped part 103, causes the resin therein to take longer to harden. Another reason is that since the sprue 104, with its thin and long structure, tends to discourage the resin therein from getting extracted therefrom, the sprue temperature is brought lower than that of the mirror plates 101 and 102 so that the resin within the sprue is cooled quickly therein and detached easily therefrom.
At the moving-side mold center is a complex combination of an ejector pin 115, a punch 116, a punch sleeve 117 and an ejector sleeve 118, all surrounding a cold slug well 119. The cold slug well 119 traps the molten resin slug that tends to occur at the nozzle tip when the resin is being filled. For the same reason explained in connection with the sprue 104, the cold slug well 119 is also set for a temperature lower than that of the mirror plates 101 and 102 using a cooling circuit formed between the punch 116 and the punch sleeve 117. This arrangement encourages the thick resin portion to cool and harden quickly.
A disc substrate is formed using the metal molds of the above-described structure as follows: A plastic resin is melted (polycarbonate resin at 280.degree. to 340.degree. C.) in a heated cylinder 112. The molten resin is injected from the tip of the nozzle 113 through the sprue 104 into the metal molds. At this time, the cooled resin slug at the tip of the nozzle 113 is trapped by the cold slug well 119 provided on the moving-side metal mold side. Most of the remaining resin fills up the disc-shaped cavity 103 between the fixed-side mirror plate 101 and the moving-side mirror plate 102, the mirror plates being set for a temperature range of about 100.degree. to 130.degree. C. Simultaneously, the stamper stamps the pits and guide grooves onto the moving-side resin surface. After filling, the punch 116 and punch sleeve 117 of the moving-side metal mold integrally protrude into the fixed-side metal mold side, forming the center hole of the disc and causing the resin within the disc cavity to be sealed by the punch sleeve 117. This keeps the resin pressure unchanged during the cooling period, thus preventing sink marks and poor stamping from occurring.
After cooling, the air (or nitrogen gas, etc.) is bled from between the fixed-side mirror plate 101 and the die 109 so that the disc is detached from the fixed-side metal mold. The entire moving-side metal mold then retracts to open the molds. At this time, the disc and the sprue 104 must be completely detached from the fixed-side metal mold and must stick to the moving-side metal mold. Then the air is bled generally from between the ejector sleeve 118 and the stamper stop 107. When detached from the stamper, the disc is ejected by the ejector sleeve 118. The sprue 104 is pushed out by the ejector pin 115 and is taken out of the molds together with the disc.
Thereafter, the entire moving-side metal mold advances, the molds are closed, and the next cycle of injection molding commences.
As described, conventional metal molds involve a complex structure, especially at the center of the moving-side mold. Because of the need to set the sprue push 111 and punch 116 lower in temperature than the mirror plates 101 and 102, there is a very large temperature distribution particularly in those mold portions that correspond to the inner circumference region of the disc. This kind of temperature distribution significantly affects the forming of the transparent substrate. The smaller the transparent substrate to be formed, the greater the adverse influence of the temperature distribution thereon. Greater double refraction tends to occur in the inner circumference region of the substrate. Illustratively, this tendency worsens the signal quality of the magnetic optical disc. The double refraction of the transparent substrate considerably deteriorates the S/N ratio of the disc formed thereby.
Furthermore, the dimensions of the punch, punch sleeve, ejector sleeve, stamper stop and other related parts have been getting smaller, to the point where it is now becoming almost impossible to sustain necessary mechanical strengths of these parts or to machine them with sufficient precision.
Under these constraints, the prior art optical recording disc is allowed to have a recording area of only up to about 60 mm in inner diameter. There do not exist known optical discs capable of recording signals to an area 50 mm or less in inner diameter.
That is, small-diameter discs so far remain capable of providing a recording area of only limited storage capacity. For example, where the standard CD format (CD-DA format) or the expanded CD-MO format (in which both reproduction and recording are available on magnetic optical discs) is used on an eight-centimeter-diameter disc, the recording or reproduction time is as short as 20 to 22 minutes. If the disc diameter is further reduced, major constraints on the recording area close to the inner circumference make it virtually impossible, even with compressed data, to secure a recording capacity for accommodating audio signals of 60 minutes or longer.